This invention relates generally to the manufacture of a cellulose based polymeric material. More particularly it relates to a modified cellulose product manufactured by impregnating a cellulosic material with an essentially uncatalyzed polyisocyanate and then curing the impregnated material at predetermined temperatures and pressures. Resulting from the manufacturing method are relatively rigid hydrophobic sheets that can be used in flat sheet form or molded into three dimensional products. In another embodiment multiple plies of uncured impregnated cellulosic material can be laminated together during the curing process. The cellulosic raw material can be in any form such as chemical wood pulps, linerboard, paper products and the like as long as it is receptive to being impregnated with an appropriate form and amount of polyisocyanate.
Polyisocyanates have had wide study as adhesives and binders for composite wood products such as hardboard and particleboard. These products have failed to find significant acceptance in the industry for various reasons.
Yamashina et al, J. Hokkaido For. Prod. Res. Inst. (Japan), 2(3):28-32 (1988) teach impregnation of rolled paper cores with catalyzed aromatic isocyanates for improving strength. These products are used; e.g., as cores for rolls of pulp or paper. Experimental details are scanty but one table compares results using three different catalysts with a control run apparently made without any catalyst.
Others have impregnated cellulosic materials with isocyanates and then cured the composite at elevated temperatures and pressures. For example, British Pat. No. 1,585,074 describes treatment of cellulosic materials, such as groundwood and kraft pulps, with solutions of blocked or capped isocyanates and a catalyst to produce moldable products having improved strength. The chemistry of the system must be selected so that the solvent used for applying the material will evaporate well below the dissociation temperature of the blocked isocyanate. Apparently one reason for the choice of a blocked isocyanate was to overcome problems of reaction with residual moisture that might be present in the fiber. This patent gives a good historical background of isocyanate-cellulose reaction products.
Published U.K. patent application No. 2,112,791 discloses a similar process but teaches that certain conditions must be met and, importantly, that a catalyst is essential. The moisture content is also apparently a critical condition as the disclosure states it should not exceed 5% and most preferably should not be more than 1 or 2% by weight. Conditions stated in the British specification are relatively high pressures, in the range of 900-1600 psi (6,200-11,000 kPa), and relatively long cure times of about 10 minutes when there is 10% by weight isocyanate in the cellulosic material.
As noted, a catalyst is stated as being an essential feature of the British process. In its absence hot pressing of the impregnated cellulose was stated to result in little or no polyurethane formation, due to the unreactive nature of the hydroxyl groups on cellulose. However, the use of catalyst at elevated temperature facilitates this reaction. The inventor notes the possible health hazard of using the blocked isocyanates described in his earlier noted British patent, and comments further on other disadvantages.
The present process represents a significant improvement over that taught in the above-noted British patent. It enables production of products at much shorter cure times, up to a full order of magnitude, and at lower pressures. Moisture content of the cellulosic sheets can be higher than that generally accepted in the British process. Storage life of the impregnated product is acceptable and physical properties of the ultimate products are superior.